AB1450 CHARACTERISTICS, OUTCOME AND FACTORS ASSOCIATED WITH EVOLUTION TO MULTIPLE MYELOMA AND SURVIVAL IN PATIENTS WITH SOLITARY BONE PLASMACYTOMA.
Authors: S. Ascione, S. Harel, F. Besson, R. Belkhir, B. Royer, B. Arnulf, X. Mariette, R. Seror
Journal: Annals of the Rheumatic Diseases (2022)

Background Solitary bone plasmacytoma (SPB) is a rare malignancy whose main risk is progression to multiple myeloma (MM). Objectives To describe the characteristics and outcome of patients treated for SBP and to identify factors associated with progression to MM and death. Methods This retrospective study was conducted between 1992 and 2020. Patients were included if they met the International Myeloma Working Group (IMWG) criteria for the diagnosis of SBP (1). Treatment response was assessed according to the IMWG criteria (2) for patients with detectable monoclonal protein and according to radiological response only for non-secretory SBP. Hazard ratios (HR) and their 95% confidence intervals (CI) were estimated by Cox proportional hazards models, adjusted for potential confounders. When available, 18 F-fluorodeoxyglucose positron emission tomography/computed tomography ( 18 F-FDG PET/CT) were reviewed by two experienced nuclear physicians. Results A total of 77 patients diagnosed with SBP were included. The median age was 59 years [range: 27-89]. Median follow-up duration was 87.1 months [range: 1.6-306.8]. At the end of the follow-up, 45 (58.4%) patients had developed a MM and 13 (16.9%) deaths occurred, including 10 patients with progression to MM. Five-year myeloma-free survival and overall survival were 47.9% and 86.8% respectively. All patients received radiotherapy with a median dose of 45 Grays [range: 30-55]. Chemotherapy was prescribed in 32 (41.6%) patients, concomitant to radiotherapy in 8, because of high risk of local extension, or adjuvant in 24 because of persistent disease after radiotherapy. Chemotherapy included mainly immunomodulatory drugs (28/32 (87.5%)) and/or proteasome inhibitors (11/32 (34.4%)) based combinations. Adjuvant chemotherapy after radiotherapy was associated with a reduced risk of MM compared to patients treated by radiotherapy alone, in a multivariate model adjusted for potential confounding factors, including response after radiotherapy alone (adjuvant chemotherapy versus no chemotherapy, HR 0.30, 95%CI [0.14-0.64]). Response to the first line of treatment (radiotherapy +/- chemotherapy) was the main factor associated with progression to MM (complete response versus partial response or progressive disease, HR 0.25, 95%CI [0.11-0.59]) and death (HR 0.22, 95%CI [0.05-0.99]). A non-significant trend between bone marrow plasmacytosis ≥ 5% at diagnosis and higher risk of MM was observed (≥ 5% versus &lt;5%, HR 2.40, 95%CI [1.00-5.74]). 18 F-FDG PET/CT images were available for review in only 36/77 patients. A significant decrease of 18 F-FDG uptake after treatment was observed whereas the lesion size was unchanged. No association was observed with the risk of MM but the limited available images may have resulted in underpowered analyses. Conclusion In this large study of patients with SBP, we observed that even though mostly prescribed for insufficient response to radiotherapy alone, treatment with adjuvant chemotherapy was associated with a decreased risk of evolution to MM, suggesting that this treatment might be systematically prescribed in addition to radiotherapy alone. These observations need to be confirmed and justify conduction of a prospective trial evaluating adjuvant chemotherapy in patients with SBP. References [1]Rajkumar SV, Dimopoulos MA, Palumbo A, Blade J, Merlini G, Mateos M-V, et al. International Myeloma Working Group updated criteria for the diagnosis of multiple myeloma. Lancet Oncol. nov 2014;15(12):e538-548. [2]Kumar S, Paiva B, Anderson KC, Durie B, Landgren O, Moreau P, et al. International Myeloma Working Group consensus criteria for response and minimal residual disease assessment in multiple myeloma. Lancet Oncol. août 2016;17(8):e328‑46. Acknowledgements The authors are indebted to all participants for their continued participation. Disclosure of Interests None declared